1. Field
The present application relates to power amplifiers. More in particular, it relates to methods and devices for improving linear power amplifier efficiency.
2. Description of Related Art
Designing electronic circuits, in particular amplifiers, according to desired specifications can involve trying to balance efficiency and linearity. In amplifier circuits, the balancing of efficiency and linearity can be achieved with feedback circuits. However, temperatures of the amplifiers can vary from device to device, and such variations in the temperature can vary the performance of the device.
Efficiency of an amplifier can be improved by introducing a feedback. FIG. 1 shows an example of a power amplifier 10 with a feedback resistor 11 to improve efficiency of the power amplifier 10. The feedback resistor 11 connects the output 14 of the power amplifier 10 to the input 13 of the power amplifier.
Given that temperature, and thus performance of a power amplifier changes from device to device, there is a need to provide control of the feedback. Additionally, there is a need to control linearity of the power amplifier, especially in terms of adjacent channel power (ACP), which is to be minimized. See, for example, RF Power Amplifiers for Mobile Communications by P. Reynaert and M. Steyaert, Springer Verlag (2010), Sec. 7.2.3 page 217, incorporated herein by reference.
By way of example, FIG. 2 shows a spectral diagram of output power from the power amplifier(s), shown as Pout, as a function frequency, shown as f. In designing RF power amplifiers, the person skilled in the art would appreciate that output power is desirable in selected bandwidth regions, such as the region shown between ωt and ω2. On the other hand, output power is not desirable in other regions, such as in the adjacent channels, shown as the regions to the left of ω1, and the regions to the right of ω2. In other words, ACP should be kept as low as possible in order to avoid leakage into adjacent channels, which can violate design specifications, which in turn, can violate, for example, Federal Communications Commission (FCC) regulations by transmitting in unauthorized channels.